In recent years, there has been renewed emphasis on impulse radio applications, including those related to Ultra-wideband transmissions. Ultra-wideband (UWB) is an emerging technology for short-range, high-speed wireless communications and other RF transmissions. In 2002, the Federal Communications Commission (FCC) authorized the unlicensed use of UWB devices on the spectrum between 3.1 GHz and 10.6 GHz. To be FCC-compliant, a UWB signal must have a bandwidth greater than 500 MHz, or a fractional bandwidth greater than 20%, and meet an FCC-specified power spectral mask.
Several photonic techniques are known for generating UWB signals. According to these photonic techniques, some or all of the signal generation process is performed on an optical or photonic signal. The photonic signal is ultimately converted to a radio frequency signal suitable for wireless transmission. This conversion is often performed with a photodetector, which is often located remotely via an optical fiber. The typical impulse is a Gaussian-monocycle or Gaussian-doublet pulse waveform. Many of the known photonic techniques suffer from disadvantages of complexity, size, and/or impulse distortion. Complexity results from the use of high-speed electronic and photonic components operating at the multi-GHz bandwidth of the impulses.
One type of known photonic UWB impulse method converts a phase-modulated optical signal to an intensity-modulated optical signal. According to this method, a high speed electronic pulse generator generates an electrical Gaussian pulse. This electrical pulse is provided to a high-speed optical phase modulator, which outputs a Gaussian-pulse phase modulated optical signal. An optical frequency discriminator receives the phase modulated optical signal and converts it to a Gaussian-monocycle or Gaussian-doublet intensity modulated optical pulse and also shifts the modulation power spectrum to the FCC UWB passband. The frequency discriminator is either an optical bandpass filter or a dispersive device such as a dispersive fiber or a chirped Bragg grating. Prior to wireless transmission, this optical pulse is converted to an electrical pulse using a photodetector.
The phase-modulation to intensity-modulation method described above suffers from several disadvantages. First, the high-speed electronic pulse generator and optical phase modulator, along with an additional optical modulator needed in most cases to encode data onto the impulses, add complexity and expense to the overall system. Additionally, implementing the common pulse position modulation (PPM) technique with a phase modulation to intensity-modulation system is non-trivial. For example, the PPM signal is often generated by a high-speed pulse selector (either electrical or optical) that selects from multiple impulses generated, either electrically or optically, at the PPM slot rate. The power, complexity and speed requirements of the system increase in proportion to the PPM order. Further, the system is sensitive to fiber transmission distortions (i.e., in applications where the photodetector is remote from other components of the system and connected by a length of optical fiber). Fiber propagation of the narrow pulses can result in waveform distortions due to fiber dispersion. Temperature variations and/or mechanical vibrations in the fiber transmission line produce phase and polarization fluctuations of the transmitted light, which can appear as noise and distortion after photodetection. Also, set-ups that utilize a dispersive device are sensitive to the length of any fiber used to distribute the signal. If that fiber length is changed, the frequency response of the system also changes, causing distortion to the UWB impulses.
Another known UWB impulse technique utilizes photonic microwave delay-line filters. According to this technique, a high-speed electronic pulse generator is similarly used to generate a Gaussian electrical pulse that, in turn, drives a high-speed optical intensity modulator. A multi-tap photonic microwave delay-line filter converts the Gaussian-pulse modulation into either Gaussian-monocycle or Gaussian-doublet modulation. These techniques, also, suffer from several disadvantages. High-speed electronic pulse generators and optical modulators are still required. Also, to realize a photonic delay-line filter with the necessary negative tap, special designs are required (e.g., cross-gain modulation in a semiconductor optical amplifier, cross-polarization modulation, or phase inversion in a Mach-Zehnder modulator). Further, the technique is susceptible to many of the fiber-transmission distortions and data modulation difficulties described above.
An additional UWB impulse technique is based on optical pulse shaping. According to this technique, the spectrum of a pulsed laser is sculpted with an optical spectral shaper to resemble a desired temporal pulse profile. Wavelength-to-time mapping by a dispersive device (e.g., a dispersive fiber or chirped Bragg grating) translates the shaped spectrum into time. The width of the impulses equals the product of the optical bandwidth and the dispersion of the dispersive device. One common type of spectral shaper uses a diffraction grating to project the laser spectrum upon a spatial light modulator (SLM), which then tailors the spectrum. Again, this method suffers from several disadvantages. For example, many of the disadvantages regarding data modulation and fiber-transmission distortions described above apply equally to optical pulse shaping techniques. Further, free-space implementation of the optical spectral shaper makes optical pulse shaping systems bulky and complicated.